There are many instances in which air can contaminate an arterial blood gas sample. For example, aspiration of a sample through a needle or the incomplete filling of a syringe may draw air into the syringe. Because the purpose of withdrawing the blood is to evaluate the patient's blood gas levels (e.g., carbon dioxide and oxygen) or variables which depend upon blood gas levels (e.g., pH), the introduction of air into the sample would serve to alter the true concentrations in the blood and cause subsequent analysis of data to be misleading.
One technique for removing air from a freshly-filled syringe is to tilt the syringe upward so that air bubbles rise to the top, cradle the open-ended head of the syringe (called the `luer`) with a piece of gauze, and advance the syringe plunger so that air is expelled. Although this technique works well in removing air from the syringe, it may also cause some blood to be expelled. For example, blood may be expelled either if the plunger is pushed too aggressively or if blood is trapped by capillarity in the uppermost portion of the luer. Thus, the present air removal technique poses an unacceptable risk in exposing workers to blood which may contain any number of biohazards.
Most prior art dealing with the air-contamination problem concentrated on expelling contaminating air from the syringe while the syringe was being filled with the patient's blood (Bailey, U.S. Pat. No. 3,978,846; Rattenborg, U.S. Pat. No. 4,340,067; These two systems incorporate hydrophilic filters into the body of the syringe. When dry, a hydrophilic filter allows air to pass through it and out of the syringe. The syringe-filter system fills with blood because a pressure differential between the luer opening and the filter is created by the patient's arterial pressure. This differential helps force air through the filter and out of the syringe. When all of the air is expelled, the leading edge of the blood contacts the filter. When wetted by the blood, the hydrophilic nature of the filter causes it to expand and prohibit passage of both air and liquid. The utility of these hydrophilic systems, however, does not extend to post-filling contaminations because the already-wetted filter will no longer pass air and so it can not be utilized to purge air introduced at later times.
In U.S. Pat. Nos. 4,769,026 and 4,775,376, Strung discloses devices that address the problems of post-filling contamination. These devices utilize separate containers equipped with hydrophobic filters. Hydrophobic filters allow for the passage of air but not liquid. Once the container and syringe are attached in an airtight fit, air and blood from the syringe are injected into this separate container by advancing the syringe plunger until no more air remains in the syringe. There are, however, two weaknesses to this type of system. First, the device's filter will always permit air passage, even when wet. Thus, preparation, transport and handling of the syringe-needle-device unit may engender the threat of air re-entry through the filter. Second, the device uses a rubber stopper to connect the needle of the syringe and the barrel of the device. The disadvantage of this system is that the needle is still connected to the syringe. Thus, preparation, transport, handling and management of the unit may pose unacceptable risks of needle prick.